Solubility Equilibrium Notes

7/30/2019 Solubility Equilibrium Notes

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Solubility Equilibrium Notes

has to be low soluble a saturated solution of any ionic compound in a closed system is an example of a chemical

equilibrium:

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Ksp is the solubility product constant: largeKsp high solubility smallKsp low solubility

all ionic compounds with very low solubility haveKsp values eg. TheKsp for limestone, CaCO3, is 4.8 109. Calculate its solubility.

R + I 0 0

C x (x = solubility) +x +x

E x x

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eg.Ksp = 8.3 109 for PbI2. What is the [Pb2+] in a saturated solution of PbI2?R + I 0 0

C x +x +2x

E x 2x

Note: saturated concentration of Pb2+ is the same as the solubility of PbI2.

Precipitation Calculations

1. A solution of AgNO3(aq) is mixed with a solution of NaCl(aq). Will a precipitate form?According to our solubility table, we would expect AgCl(S) to form. However, this table deals with

solutions of concentration greater than 0.1 M (ie. if the [AgNO3(aq)] and the [NaCl(aq)] were both

greater than 0.1 M, we know a precipitate of AgCl(S) will form. If one or both concentrations are

less than 0.1 M, we must useKsp.


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2. Predict if a precipitate will form when 20.0 mL of 0.0020 M Ag+ is added to 30.0 mL of0.0020 M Cl:

( )

( ) * not at equilibrium

3. What concentration of OHis required to just start precipitation of Fe(OH)3 from a solutioncontaining 2.0 106 M Fe3+?Ksp for Fe(OH)3 is 6.0 10-38.

Common Ion Effect

eg. PbCl2 is less soluble in a solution of NaCl than it is in pure water. The solution of NaClcontains a common ion (Cl).

The presence of a common ion will reduce the solubility of a particular substance in aparticular solution.

1. CaSO4 is less soluble in a solution of Na2SO4 than it is in pure water (SO42 is common toCaSO4): The presence of SO42 from the Na2SO4 shifts this equilibrium to the left, decreasing the

solubility of CaSO4 in this solution (note: the value ofKsp is unchanged).

2. CaSO4 is added to 1.00 L of a 0.100 M solution of Na 2SO4 until no more will dissolve. IfKspfor CaSO4 is 2.45 105, what is the concentration of Ca2+ in the saturated solution?

R + I 0 0.100

C x +x +x

E x 0.100 + x

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Alternate Solution


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[] becauseKsp is small, assume thatxis very small and is negligible compared to 0.100:

Check Assumption:

3. Name 2 compounds that, when added to a solution, will decrease the solubility of CaCl2. NaCl or Ca(NO3)2 or any other soluble salt containing Cl or Ca2+. What about CaCO3?

(hint: check your solubility table). CaCO3 has low solubility, so it does not produce

enough Ca2+ to have any significant effect.

Qualitative Analysis

1. A solution is known to contain Pb2+, Cu2+, and Ca2+ ions. Devise a method for separatingthese ions (assume concentration > 0.1 M).

one answer:i. Add KI and remove precipitate (I is soluble with Cu2+ and Ca2+, but not with Pb2+):

ii. Add K2S and remove precipitate (S2 is soluble with Ca2+, but not with Cu2+):

iii. Ca2+ is left in the solution.2. Determine the identity of the unknown cation in the following Solution X:

adding SO42 does produce a precipitate adding S2 does not produce a precipitate adding OH does not produce a precipitate

Answer: Sr2+3. If a 0.10 M solution of AgNO3 is used to detect the presence of Cl ion in water samples, what

is the minimum concentration of Cl that can be detected?

M

Applications of Solubility

Chloride Ion Titration

The concentration of chloride ion in a water sample is determined. Adding Ag+ to the water sample will cause AgCl(s) to precipitate: Enough Ag+ is added to react with all of the Cl. The moles of Ag+ added equal the moles of Cl that were present in the water sample and

[Cl] can be calculated.


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eg. 25.0 mL of a water sample was titrated with 0.500 M AgNO3(aq) using sodium chromate as an

indicator. At the equivalence point, 26.8 mL of AgNO3 solution had been added. What was the

concentration of Cl in the water sample?

Softening Hard Water

Hard water is caused by the presence of metal ions such as Ca2+ and Mg2+ (also Fe2+). These ions react with soap molecules and inhibit their function. Slow-moving ground water (such as well water) is most likely to be hard (more time for ions

to dissolve into the water).

To soften water, washing soda may be added (Na2CO3). The washing soda precipitates the metal ions:

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Solubility Equilibrium Notes